Laser peening (also known as laser shock processing or laser shock peening) is an innovative surface treatment for improving the fatigue strength and damage tolerance of metal parts. Laser peening drives high amplitude shock waves into a part surface using high intensity laser pulses. The shock waves are used to develop deep compressive residual stresses in the surfaces of fatigue-prone parts. Typically, these stresses penetrate five to ten times deeper than conventional metal shot peening. These compressive surface stresses inhibit the initiation and propagation of fatigue cracks.
Laser peening has been particularly effective at preventing fatigue failures in aircraft engine metal alloy fan and compressor blades. However, the application of laser peening is much broader. The application can encompass aerospace structures, helicopter gears and propulsion components, automotive parts, orthopedic implants, tooling and dies, and numerous other military and industrial parts prone to metal fatigue failures.
Before laser peening, an overlay coating, which may be substantially opaque to the laser beam, may be applied to the part surface being treated. An additional layer, which may be substantially transparent to the laser beam, may be placed over the opaque overlay. The transparent layer may also be applied directly to the part surface, without the application of an opaque layer. The opaque overlay may be, for example, black paint, or tape. The transparent overlay may be, for example, flowing water.
Suitable laser peening systems, apparatuses, and processing conditions are disclosed in, for example, one or more of U.S. Pat. Nos. 5,741,559, 6,191,385, 6,373,876, and 7,268,317, each of which is incorporated by reference in its entirety.
The laser pulses pass through the transparent overlay and strike the opaque overlay, causing the opaque overlay to vaporize. The vapor absorbs the remaining laser energy and produces a rapidly expanding plasma plume. Because the expanding plasma is confined momentarily between the surface of the part and the transparent overlay, a rapidly rising high-pressure shock wave is created, which propagates into the part. When the peak stress created by the shock wave is above the dynamic yield strength of the metal part, the metal yields, and the metal is “cold worked” or plastically deformed on, and just under, the surface. This plastic deformation results in compressive residual stresses in the surface of the part. The depth and magnitude of the residual stresses depend upon the magnitude and rate of attenuation of the shock wave as it passes through the surface layer, the material properties, and the processing conditions specific to the application. Compressive residual stresses typically extend as deep or deeper than about 0.040 to about 0.060 inches (about 1.0 to about 1.5 mm) into the surface, and can approach the yield strength of the material.
Laser peening typically requires line-of-sight access to the surface of the workpiece to be treated. In laser peening operations, it is sometimes necessary to laser peen surfaces that may not be readily processed using traditional laser peening apparatuses. For example, some surfaces are hidden, or out of direct line-of-sight, and as such are not able to be exposed to a laser beam generated from outside the workpiece. Such surfaces may include, for example, inside holes, notches, grooves, internal cavities, dovetail joints, and the like. Previous attempts at laser peening such hidden surfaces have required the insertion of a reflective element into the cavity adjacent to the hidden surface, and directing a laser pulse onto the reflective element, which redirects the pulse onto the hidden surface. However, this method requires additional steps and, thus, additional time to laser peen hidden surfaces. What is needed is a laser peening apparatus that is capable of accessing these hidden surfaces and directly applying a laser pulse thereto.